In inputting or outputting a microwave signal to/from a device such as a superconducting filter or a low-noise amplifier, which is operated at a very low temperature where a temperature difference from an ordinary temperature (around 300 K) is 150 kelvin (K) or more, microwaves need to be transmitted between the ordinary temperature and the very low temperature. To maintain the very low-temperature environment, it is desirable to suppress a heat inflow from an outside. Therefore, a thermal insulation waveguide that transmits the microwaves with a low loss while suppressing the heat inflow is necessary. As such a thermal insulation waveguide, a method of using a member having poor thermal conductivity for the thermal insulation waveguide or a transmission path provided with a gap in the middle of a waveguide tube has been proposed.
As a method of transmitting a signal having high thermal insulation, a method of achieving both of thermal insulation and a low loss property by dividing the waveguide tube and having a gap to suppress heat intrusion, and providing a reflecting plate to suppress deterioration of a pass characteristic of a high frequency signal, and a method of configuring a transmission line using a member having poor thermal conductivity, and loading the transmission line between a high-temperature unit and a low-temperature unit to suppress heat intrusion, are known.
In a case of using a structure that achieves both of the thermal insulation and the low loss property by dividing the waveguide tube and providing the gap to suppress the heat intrusion, and providing the reflecting plate to suppress deterioration of the pass characteristic of the high frequency signal, there is a problem that the structure has a large sectional area and is tall, and is thus not suitable for downsizing, due to the limitation on the structure by use of the waveguide tube. Further, to decrease in the sectional area of the waveguide tube, a method of loading a dielectric inside the waveguide tube (dielectric mounted-type gap waveguide tube) is also known. However, in this case, a heat inflow amount is increased due to heat radiation between dielectrics having larger emissivity than typical metals. Further, in a case of using the waveguide tube as a waveguide, there is also a problem that a loss is increased due to mismatch in converting the waveguide tube into a planar circuit system such as a filter or a low-noise amplifier.
Meanwhile, in the method of suppressing the heat intrusion using the member having poor thermal conductivity for the transmission path, the limitation on the structure is smaller than the waveguide tube structure. However, the member having poor thermal conductivity is used as a member that configures the transmission path. Therefore, there is a problem that a dielectric loss in a high frequency is typically large, and thus the pass characteristic of the signal is deteriorated and the low loss property is deteriorated. Further, in a case of configuring a microstrip or a coplanar line, a ground surface is required, and thus there is a problem that the heat intrusion through a ground layer becomes large. Therefore, when the transmission path is used to configure a circuit having a low loss property such as a superconducting device, there is a problem that a loss in the transmission path of the circuit becomes large, and superconducting characteristics cannot be fully used. Therefore, a small thermal insulation waveguide having a small heat inflow amount and a small microwave transmission loss is required.